Explanation:

When importing data like electricity bills into Salesforce, and you want to avoid duplicating records, the best approach is to use:

Update/Insert (Upsert) with Data Import Wizard

✅ This method checks for existing records based on a unique identifier (like Record ID or external ID).
✅ If a matching record exists → it updates it.
✅ If no matching record is found → it inserts a new one.
✅ This is the best way to safely import bulk data without duplication.

❌ Why the other options are incorrect:

A. Merge Leads Function
❌ Only applies to Leads, not custom objects like Energy Use Records or imported bills.

B. Validation rules
❌ These ensure data quality, not deduplication. They can't prevent record duplication on their own.

C. Insert new records with Data Loader
❌ This will add new records regardless of duplicates, potentially creating many copies.